1. Field of the Invention
The present invention relates to a method and apparatus for forming refractory metal films at contacts of a substrate, and particularly to a method and apparatus for forming refractory metal films that do not easily separate from a substrate on which the films are formed.
2. Description of the Prior Art
FIGS. 1a to 1c are views showing a prior art method for forming contacts on a semiconductor substrate between an impurity diffusion layer and electrode wiring of the substrate.
As shown in FIG. 1a, a semiconductor substrate 1 is provided with element separation regions 2. A silicon oxide film 3 of about 0.8 .mu.m in thickness is formed on the substrate 1 by using a vapor deposition technique. A numeral 4 represents a diffusion layer formed by using an ion implantation technique and having conduction type opposite to that of the substrate 1.
As shown in FIG. 1b, a contact hole 5 is opened at a required position of the silicon oxide film 3 according to a known process such as a photoetching process and a reactive ion etching process to provide an access to the diffusion layer 4. Then, as shown in FIG. 1c, a wiring metal film 6 of about 0.8 .mu.m in thickness made of, for example, Al/Si alloy is formed by using a spattering technique, etc. The wiring metal film 6 is patterned by using a known process such as the photoetching process and the reactive ion etching process to form a circuit.
The contact forming method explained in the above will hereinafter be referred to as the first contact forming method.
With improvements in the integration of circuits and the miniaturization of elements, the first contact forming method is currently having two problems.
The first problem is that substances such as Si contained in the wiring metal film 6 precipitate on the bottom of the contact hole 5. Due to the preciptitation, an effective contact area of the contact hole 5 reduces to increase contact resistance. If many substances precipitate in the contact hole 5, conduction of the contact is seriously deteriorated.
The second problem occurs in forming very fine elements. Namely, it occurs when a ratio of diameter to depth (aspect ratio) of the contact hole 5 is large. If the aspect ratio is large, or if the contact hole is very small and deep, a wiring metal film of the sputtering process is not sufficiently formed to reach the bottom of the hole. This insufficient formation of film in the contact hole is the second problem and causes low-step coverage by a shadowing effect. Due to the shadowing effect, metal particles of the wiring metal film deposit to a upper corner of the hole to block an opening of the hole. The shadowing effect may cause a disconnection failure of the contact.
These two problems may seriously reduce the reliability of semiconductor elements.
To solve the problems of the first contact forming method, a method has been proposed to use the chemical vapor deposition technique with refractory metal halides to selectively embed refractory metal films in contact holes formed on a diffusion layer or on electrodes.
For example, a mixture of refractory metal halide gas such as tungsten hexafluoride (WF.sub.6) gas and silane (SiH.sub.4) gas is introduced into a reactor vessel in which a substrate is positioned, to selectively embed refractory metal films, i.e., tungsten (W) films in contact holes of the substrate according to the chemical vapor deposition technique.
Namely, as shown in FIG. 2a, the same technique as that used for the first contact forming method is used to form a contact hole 5 on a semiconductor substrate 1. The substrate 1 is placed in the reactor vessel whose inside pressure is reduced. In FIGS. 2a to 2c, the same reference marks as those shown in FIGS. 1a to 1c represent like parts. A mixture of WF.sub.6 gas and SiH.sub.4 gas is introduced into the reactor vessel at about 400.degree. C. to form a tungsten (W) film 9 only in the contact hole 5. After that, the sputtering technique is used to coat the substrate 1 with wiring metal film 6 of Al/Si alloy of about 0.8 .mu.m in thickness. The wiring metal film 6 is patterned with the same technique as that of the first contact forming method to form a circuit.
This latter method is hereinafter referred to as the second contact forming method.
The second contact forming method may solve the two problems of the first contact forming method. Since the wiring 6 made of Al/Si alloy is not directly contacting with the substrate 1, silicon (Si) does not precipitate in the contact hole 5. Not like the sputtering technique, the tungsten film 9 is satisfactorily embedded on a diffusion layer 4 due to the reaction of the gases so that no stepped portions will be formed at the contact, thus preventing a disconnection failure at the contact from occurring.
However, after testing the second contact forming method several times, it has been found that adhesion between the silicon substrate 1 and the tungsten film 9 is very weak. For example, in the case of a tungsten film deposited for 1 .mu.m or more, a separation 10 shown in FIG. 2c has been caused due to thermal and/or intrinsic stress to cause a contact fault. In the case of a tungsten film deposited for less than 1 .mu.m, the same separation and contact failure have occurred after a thermal cycle test carried out in the temperature range of +200.degree. C. to -100.degree. C.
Therefore, even with the second contact forming method, it is difficult to maintain the reliability of semiconductor devices. The problems mentioned above are very serious in integrating semiconductor circuits.
In the case of forming refractory metal films not as contacts but as, for example, wiring on a substrate made of silicon, etc., the separation may occur between the wiring and the substrate when the substrate is exposed to hot water or liquid nitrogen.
As described in the above, in forming electrodes and wiring on a substrate, there is a need to solve the problem of insufficient adhesion between refractory metal films and the substrate.